This invention relates to the production of dichlorinated paraxylene, particularly to a process which results in the predominant production of the 2,5-dichloro-p-xylene isomer.
Various processes are known in the prior art for halogenating aromatic compounds. Generally, the known processes result in the production of a mixture of halogenated products. A number of processes are disclosed for chlorinating p-xylene. These processes produce various mixtures of mono-, di-, tri-, and in some instances tetrachlorinated products. While the product can be separated into its various components, it would be desirable to have a process for the selective production of 2,5-dichloro-p-xylene, since this product is highly useful as an intermediate in the production of pesticides.
The typical prior art known processes for producing chlorinated xylenes involve reacting xylene and chlorine in an organic solvent medium in the presence of a catalyst. U.S. Pat. No. 3,002,027 discloses a process wherein chlorine and p-xylene are reacted in glacial acetic acid as a solvent. The glacial acetic acid acts as a "chlorination stopping agent" to prevent the substitution of more than 2 chlorine atoms on the p-xylene nucleus. The reaction product consists of a mixture containing both 2,3-dichloro-p-xylene and 2,5-dichloro-p-xylene in addition to 2-chloro-p-xylene. The 2,5-dichloro-p-xylene is recovered from the crude reaction product by pouring the product into water followed by re-crystallization of the precipitate from a suitable solvent such as an organic alcohol or acetic acid. In separating the crude product, however, some of the desired 2,5-dichloro-p-xylene is lost in the oily byproduct phase.
U.S. Pat. No. 3,035,103 discloses a process wherein p-xylene is reacted with chlorine in the presence of a catalytic amount of ferric chloride to produce a mixture of 2,5-dichloro-p-xylene, 2,3-dichloro-p-xylene, 2,3,5-trichloro-p-xylene, and 2,3,5,6-tetrachloro-p-xylene. The mixture is distilled to separate the dichlorinated compounds from the tri- and tetrachlorinated compounds. The 2,5-dichloro-p-xylene must be separated from the 2,3-dichloro-p-xylene by contacting the mixture with a solvent in which only the 2,3-dichloro-p-xylene is soluble.
U.S. Pat. No. 4,010,214 discloses a process for selectively chlorinating p-xylene to produce predominanty 2,5-dichloro-p-xylene which comprises reacting chlorine and p-xylene in the presence of a catalyst system comprising halides of iron and/or antimony in conjunction with a co-catalyst which is an organic sulfur compound characterized by divalent sulfur. The crude chlorination product thus produced is said to typically contain 65 to 80 percent by weight 2,5-dichloro-p-xylene, 3 to 20 percent by weight 2,3-dichloro-p-xylene, 0 to 6 percent by weight of 2-chloro-p-xylene, 0 to 8 percent by weight 2,3,5-trichloro-p-xylene and traces of 2,3,5,6-tetrachloro-p-xylene. In the working examples of the patent, the concentrations of 2,3-dichloro-p-xylene in the crude product ranged from 11.2 to 19.1 weight percent. The 2,5-dichloro-p-xylene must be separated from 2,3-dichloro-p-xylene in a manner similar to that disclosed in U.S. Pat. No. 3,035,103.
Despite the ingenuity displayed in the prior art, a simple and direct method is still sought for the selective production of 2,5-dichloro-p-xylene of sufficiently high purity that the product is useful as an intermediate in the production of pesticides without the need for complicated separation steps.